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United Airlines says it will more strictly enforce its carry-on policy

The airline said the move was prompted by complaints about bin hogging

The policy hasn't changed but more oversized bags may end up checked for $25

Attention passengers: United Airlines is cracking down on travelers with oversized carry-on luggage.

The airline says it is motivated not by money -- the airline charges $25 for checked luggage -- but by complaints that some passengers are hogging space in the overhead bins.

To address those gripes, United recently e-mailed frequent fliers reminding them of the size restrictions for carry-ons. And employees were told to keep an eye out for large carry-on bags.

"This is solely about ensuring the greatest number of customers with standard-sized carry-ons are able to store them onboard," a spokesman said.

The airline stressed it is not changing its policy but is trying to more strictly enforce it. If your bag doesn't fit in the bag measure at the security line, be prepared to be directed back to the ticket counter to check the bag for a $25 bag fee.

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Passengers who get through the security checkpoint with an oversized bag will not be sent back to the ticket counter, a spokesman said. Instead, the bag will be gate-checked for free.

Even if your large carry-on could technically be squeezed into the bin, United said it takes away space from other customers if it's not within the size restrictions (9 x 14 x 22 inches, including handles and wheels.)

The airline retrofitted 152 airbus planes with larger bins. They say their newer planes acquired in the past 10 years also have larger bins.

Oversized carry-ons is a frequent problem on domestic flights and less so on international flights, since most international fliers headed to a destination for a longer period usually check luggage.